arXiv subjects

Joshua N. Winn

Publications and source records attributed to Joshua N. Winn.

At least 433 records · Page 24Linked to original sources

arXiv2009

The Transit Light Curve Project. XII. Six Transits of the Exoplanet XO-2b

We present photometry of six transits of the exoplanet XO-2b. By combining the light-curve analysis with theoretical isochrones to determine the stellar properties, we find the planetary radius to be 0.996 +0.031/-0.018 rjup and the planetary mass to be 0.565 +/- 0.054 mjup. These results are consistent with those reported previously, and are also consistent with theoretical models for gas giant planets. The mid-transit times are accurate to within 1 min and are consistent with a constant period. However, the period we derive differs by 2.5 sigma from the previously published period. More data are needed to tell whether the period is actually variable (as it would be in the presence of an additional body) or if the timing errors have been underestimated.

astro-ph.SR↗

arXiv2009

Near-infrared transit photometry of the exoplanet HD 149026b

The transiting exoplanet HD 149026b is an important case for theories of planet formation and planetary structure, because the planet's relatively small size has been interpreted as evidence for a highly metal-enriched composition. We present observations of 4 transits with the Near Infrared Camera and Multi-Object Spectrometer on the Hubble Space Telescope, within a wavelength range of 1.1--2.0 $μ$m. Analysis of the light curve gives the most precise estimate yet of the stellar mean density, $ρ_\star = 0.497^{+0.042}_{-0.057}$ g cm$^{-3}$. By requiring agreement between the observed stellar properties (including $ρ_\star$) and stellar evolutionary models, we refine the estimate of the stellar radius: $R_\star = 1.541^{+0.046}_{-0.042}$ $R_\sun$. We also find a deeper transit than has been measured at optical and mid-infrared wavelengths. Taken together, these findings imply a planetary radius of $R_p = 0.813^{+0.027}_{-0.025}$ $R_{\rm Jup}$, which is larger than earlier estimates. Models of the planetary interior still require a metal-enriched composition, although the required degree of metal enrichment is reduced. It is also possible that the deeper NICMOS transit is caused by wavelength-dependent absorption by constituents in the planet's atmosphere, although simple model atmospheres do not predict this effect to be strong enough to account for the discrepancy. We use the 4 newly-measured transit times to compute a refined transit ephemeris.

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arXiv2009

Exoplanetary Spin-Orbit Alignment: Results from the Ensemble of Rossiter-McLaughlin Observations

One possible diagnostic of planet formation, orbital migration, and tidal evolution is the angle psi between a planet's orbital axis and the spin axis of its parent star. In general, psi cannot be measured, but for transiting planets one can measure the angle lambda between the sky projections of the two axes via the Rossiter-McLaughlin effect. Here, we show how to combine measurements of lambda in different systems to derive statistical constraints on psi. We apply the method to 11 published measurements of lambda, using two different single-parameter distributions to describe the ensemble. First, assuming a Rayleigh distribution (or more precisely, a Fisher distribution on a sphere), we find that the peak value is less than 22 degrees with 95% confidence. Second, assuming a fraction f of the orbits have random orientations relative to the stars, and the remaining fraction (1-f) are perfectly aligned, we find f<0.36 with 95% confidence. This latter model fits the data better than the Rayleigh distribution, mainly because the XO-3 system was found to be strongly misaligned while the other 10 systems are consistent with perfect alignment. If the XO-3 result proves robust, then our results may be interpreted as evidence for two distinct modes of planet migration.

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arXiv2009

The Transit Light Curve Project. XI. Submillimagnitude Photometry of Two Transits of the Bloated Planet WASP-4b

We present photometry of two transits of the giant planet WASP-4b with a photometric precision of 400-800 parts per million and a time sampling of 25-40 seconds. The two midtransit times are determined to within 6 seconds. Together with previously published times, the data are consistent with a constant orbital period, giving no compelling evidence for period variations that would be produced by a satellite or additional planets. Analysis of the new photometry, in combination with stellar-evolutionary modeling, gives a planetary mass and radius of 1.237 +/- 0.064 M_jup and 1.365 +/- 0.021 R_jup. The planet is 15% larger than expected based on previously published models of solar-composition giant planets. With data of the quality presented here, the detection of transits of a "super-Earth" of radius 1.75 R_earth would have been possible.

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arXiv2008

Photometric Follow-up Observations of the Transiting Neptune-Mass Planet GJ 436b

This paper presents multi-band photometric follow-up observations of the Neptune-mass transiting planet GJ 436b, consisting of 5 new ground-based transit light curves obtained in May 2007. Together with one already published light curve we have at hand a total of 6 light curves, spanning 29 days. The analysis of the data yields an orbital period P = 2.64386+-0.00003 days, mid-transit time T_c [HJD] =2454235.8355+-0.0001, planet mass M_p = 23.1+-0.9 M_{\earth} = 0.073+-0.003 M_{Jup}, planet radius R_p = 4.2+-0.2 R_{\earth} = 0.37+-0.01 R_{Jup} and stellar radius R_s = 0.45+-0.02 R_{\sun}. Our typical precision for the mid transit timing for each transit is about 30 seconds. We searched the data for a possible signature of a second planet in the system through transit timing variations (TTV) and variation of the impact parameter. The analysis could not rule out a small, of the order of a minute, TTV and a long-term modulation of the impact parameter, of the order of +0.2 year^{-1}.

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arXiv2008

A Smaller Radius for the Transiting Exoplanet WASP-10b

We present photometry of WASP-10 during the transit of its short-period Jovian planet. We employed the novel PSF-shaping capabilities the OPTIC camera mounted on the UH 2.2m telescope to achieve a photometric precision of 4.7e-4 per 1.3 min sample. With this new light curve, in conjunction with stellar evolutionary models, we improve on existing measurements of the planetary, stellar and orbital parameters. We find a stellar radius Rstar = 0.698 +/- 0.012 Rsun and a planetary radius Rp = 1.080 +/- 0.020 Rjup. The quoted errors do not include any possible systematic errors in the stellar evolutionary models. Our measurement improves the precision of the planet's radius by a factor of 4, and revises the previous estimate downward by 16% (2.5sigma, where sigma is the quadrature sum of the respective confidence limits). Our measured radius of WASP-10b is consistent with previously published theoretical radii for irradiated Jovian planets.

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arXiv2008

A Precise Estimate of the Radius of the Exoplanet HD 149026b from Spitzer Photometry

We present Spitzer 8 micron transit observations of the extrasolar planet HD 149026b. At this wavelength, transit light curves are weakly affected by stellar limb-darkening, allowing for a simpler and more accurate determination of planetary parameters. We measure a planet-star radius ratio of Rp/Rs = 0.05158 +/- 0.00077, and in combination with ground-based data and independent constraints on the stellar mass and radius, we derive an orbital inclination of i = 85.4 +0.9/-0.8 degrees and a planet radius of Rp = 0.755 +/- 0.040 R_jup. These measurements further support models in which the planet is greatly enriched in heavy elements.

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arXiv2008

Empirical Constraints on Trojan Companions and Orbital Eccentricities in 25 Transiting Exoplanetary Systems

We present a search for Trojan companions to 25 transiting exoplanets. We use the technique of Ford & Gaudi, in which a difference is sought between the observed transit time and the transit time that is calculated by fitting a two-body Keplerian orbit to the radial-velocity data. This technique is sensitive to the imbalance of mass at the L4/L5 points of the planet-star orbit. No companions were detected above 2σconfidence. The median 2σupper limit is 56 M_\earth, and the most constraining limit is 2.8 M_\earth for the case of GJ 436. A similar survey using forthcoming data from the Kepler satellite mission, along with the radial-velocity data that will be needed to confirm transit candidates, will be sensitive to 10-50 M_\earth Trojan companions in the habitable zones of their parent stars. As a by-product of this study, we present empirical constraints on the eccentricities of the planetary orbits, including those which have previously been assumed to be circular. The limits on eccentricity are of interest for investigations of tidal circularization and for bounding possible systematic errors in the measured planetary radii and the predicted times of secondary eclipses.

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arXiv2008

The Transit Light Curve Project. X. A Christmas Transit of HD 17156b

Photometry is presented of the Dec. 25, 2007 transit of HD 17156b, which has the longest orbital period and highest orbital eccentricity of all the known transiting exoplanets. New measurements of the stellar radial velocity are also presented. All the data are combined and integrated with stellar-evolutionary modeling to derive refined system parameters. The planet's mass and radius are found to be 3.212_{-0.082}^{+0.069} Jupiter masses and 1.023_{-0.055}^{+0.070} Jupiter radii. The corresponding stellar properties are 1.263_{-0.047}^{+0.035} solar masses and 1.446_{-0.067}^{+0.099} solar radii. The planet is smaller by 1 sigma than a theoretical solar-composition gas giant with the same mass and equilibrium temperature, a possible indication of heavy-element enrichment. The midtransit time is measured to within 1 min, and shows no deviation from a linear ephemeris (and therefore no evidence for orbital perturbations from other planets). We provide ephemerides for future transits and superior conjunctions. There is an 18% chance that the orbital plane is oriented close enough to edge-on for secondary eclipses to occur at superior conjunction. Observations of secondary eclipses would reveal the thermal emission spectrum of a planet that experiences unusually large tidal heating and insolation variations.

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arXiv2008

Measuring accurate transit parameters

By observing the transits of exoplanets, one may determine many fundamental system parameters. I review current techniques and results for the parameters that can be measured with the greatest precision, specifically, the transit times, the planetary mass and radius, and the projected spin-orbit angle.

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arXiv2008

Analytic Approximations for Transit Light Curve Observables, Uncertainties, and Covariances

The light curve of an exoplanetary transit can be used to estimate the planetary radius and other parameters of interest. Because accurate parameter estimation is a non-analytic and computationally intensive problem, it is often useful to have analytic approximations for the parameters as well as their uncertainties and covariances. Here we give such formulas, for the case of an exoplanet transiting a star with a uniform brightness distribution. We also assess the advantages of some relatively uncorrelated parameter sets for fitting actual data. When limb darkening is significant, our parameter sets are still useful, although our analytic formulas underpredict the covariances and uncertainties.

astro-ph↗

arXiv2008

Analytic approximations for transit light curve observables and uncertainties

The light curve of an exoplanetary transit can be used to estimate the planetary radius and other parameters of interest. Because accurate parameter estimation is a non-analytic and computationally intensive problem, it is often useful to have analytic approximations for the parameters as well as their uncertainties and covariances. Here we give such formulas, for the case of an exoplanet transiting a star with a uniform brightness distribution. When limb darkening is significant, our parameter sets are still useful, although our analytic formulas underpredict the covariances and uncertainties.

astro-ph↗

arXiv2008

A Search for Exotrojans in Transiting Exoplanetary systems

We present a search for Trojan companions to 25 transiting exoplanets. We use the technique of Ford & Gaudi 2006, in which a difference is sought between the observed transit time and the transit time that is calculated by fitting a two-body Keplerian orbit to the radial-velocity data. This technique is sensitive to the imbalance of mass at the L4/L5 points of the planet-star orbit. No companions were detected. The median 2$σ$ upper limit is 60 Earth masses, and the most constraining limit is 2.5 Earth masses for the case of GJ 436.

astro-ph↗

arXiv2008

A Precise Estimate of the Radius of HD 149026b

We present Spitzer 8 micron transit observations of the extrasolar planet system HD 149026. At this wavelength, transit light curves are weakly affected by stellar limb-darkening, allowing for a simpler and more accurate determination of planetary parameters. We measure a planet-star radius ratio of R_p/R_s = 0.05158 +/- 0.00077, and in combination with ground-based data and independent constraints on the stellar mass and radius, we derive an orbital inclination of i = 85.4 +0.9/-0.8 deg. and a planet radius of 0.755 +/- 0.040 Jupiter radii. These measurements further support models in which the planet is greatly enriched in heavy elements.

astro-ph↗

arXiv2008

Toward a homogeneous set of transiting planet parameters

With 40 or more transiting exoplanets now known, the time is ripe to seek patterns and correlations among their observed properties, which may give important insights into planet formation, structure, and evolution. This task is made difficult by the widely different methodologies that have been applied to measure their properties in individual cases. Furthermore, in many systems our knowledge of the planet properties is limited by the knowledge of the properties of the parent stars. To address these difficulties we have undertaken the first comprehensive analysis of the data for 23 transiting planets using a uniform methodology. We revisit several of the recently proposed correlations, and find new ones involving the metallicity of the parent stars.

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arXiv2008

Measurement of the Spin-Orbit Angle of Exoplanet HAT-P-1b

We present new spectroscopic and photometric observations of the HAT-P-1 planetary system. Spectra obtained during three transits exhibit the Rossiter-McLaughlin effect, allowing us to measure the angle between the sky projections of the stellar spin axis and orbit normal, λ= 3.7 +/- 2.1 degrees. The small value of λfor this and other systems suggests that the dominant planet migration mechanism preserves spin-orbit alignment. Using two new transit light curves, we refine the transit ephemeris and reduce the uncertainty in the orbital period by an order of magnitude. We find a upper limit on the orbital eccentricity of 0.067, with 99% confidence, by combining our new radial-velocity measurements with those obtained previously.

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arXiv2008

The Transit Light Curve Project. IX. Evidence for a Smaller Radius of the Exoplanet XO-3b

We present photometry of 13 transits of XO-3b, a massive transiting planet on an eccentric orbit. Previous data led to two inconsistent estimates of the planetary radius. Our data strongly favor the smaller radius, with increased precision: R_p = 1.217 +/- 0.073 R_Jup. A conflict remains between the mean stellar density determined from the light curve, and the stellar surface gravity determined from the shapes of spectral lines. We argue the light curve should take precedence, and revise the system parameters accordingly. The planetary radius is about 1 sigma larger than the theoretical radius for a hydrogen-helium planet of the given mass and insolation. To help in planning future observations, we provide refined transit and occultation ephemerides.

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arXiv2008

The Transit Light Curve Project. VIII. Six Occultations of the Exoplanet TrES-3

We present photometry of the exoplanet host star TrES-3 spanning six occultations (secondary eclipses) of its giant planet. No flux decrements were detected, leading to 99%-confidence upper limits on the planet-to-star flux ratio of 0.00024, 0.0005, and 0.00086 in the i, z, and R bands respectively. The corresponding upper limits on the planet's geometric albedo are 0.30, 0.62, and 1.07. The upper limit in the i band rules out the presence of highly reflective clouds, and is only a factor of 2-3 above the predicted level of thermal radiation from the planet.

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